Kustannusoptimoinnin menetelmät leijukerroskattiloille

In this Master of Science thesis, the operational cost structure and a method to optimize this structure are studied. The whole operational cost structure and different basis for optimization are presented. Optimization of the soot-blowing cycle was chosen as the studied method as it is a feasible target for optimization from technical point of view and it is linked to the cost structure in a straightforward manner. This thesis of focused on the most common form of fluidized bed boilers, circulating fluidized bed boilers, but the theory and studied method are for the most parts also applicable to bubbling fluidized bed boilers. Research targets of this thesis were to identify factors that form the operational cost structure of a circulating fluidized bed boiler, to identify the most critical Key Performance Indicators from cost point of view and to study the feasibility of soot-blowing optimization as a cost optimization method. This thesis is divided into theoretical and computational parts. In the theoretical part the techno-economic background of circulating fluidized bed boiler technology is introduced, including the operational cost structure of circulating fluidized bed boilers. Theory of Key Performance Indicators and state of the art references regarding soot-blowing optimization are presented in their own Sections. In the computational part the developed method for soot-blowing optimization is presented. This method was applied to six different power plants and the most cost optimal soot-blowing cycles for these power plants were calculated from history data based on the behavior of flue gas losses between soot-blowing cycles. The results clearly indicate, that power plant operators initiate soot-blowing cycles too often as in all inspected cases the calculated optimal cycles were longer than the cycles that were used on average. The calculated potential savings on a yearly level varied from 200 to 12 800 tons of fuel. These results clearly indicate, that soot-blowing optimization is a feasible cost optimization method in fluidized bed boilers

Lämmönsiirtoon pohjautuva likaantumistaipumuksen tarkastelu leijupetikattiloissa

Fly ash components in the flue gas can stick onto heat exchanger tubes and cumulate into solid deposits. This fouling phenomenon is a common issue in fluidized bed boilers, where challenging fuels with possibly high and challenging ash content are often fired in. Basic characteristics of the phenomenon are discussed briefly, followed by a short review of research on the methods of modelling and predicting fouling tendency. A method to examine fouling through heat transfer calculations was tested in this thesis. Primary aim was to verify applicability of this selected method in real large-scale boilers via performing a retrospective analysis on earlier measurement data. Reference clean state heat transfer coefficients of certain heat exchangers were compared to calculated states by using data from control system log files, and the comparisons were formulated into thermal resistances of the deposit layer. The control system log files contained data from earlier measurement campaigns. Calculated thermal resistances increased along cumulating deposition, until a cleansing soot blowing pulse is actuated. Slopes of these rising thermal resistance curves were extracted, forming estimates of fouling rates per each fouling period. Calculated thermal resistance build-ups matched soot blowing operation times well with only a few exceptions, and so the selected method seemed to express actual fouling decently in general. Calculated resistances and fouling rates were compared to other operational factors, including main steam power, fuel feed variation and measured flue gas pressure change at studied heat exchangers. Certain findings were made, even though available data was not completely sufficient. While decent correlation with slight steam power changes was not identifiable, studying the flue gas pressure change showed very evident relation with thermal resistances. Fuel mixture appeared to affect the fouling rates, but not consistently with small changes in the fuel feed. Conclusions of fouling differences between superheater and economizer temperature zones could not be made

Slaging Analysis Based on Boiler Wall Temperature at PLTU Paiton Unit 3

Slagging on the surface of boiler walls in power plants is still a serious problem which reduces thermodynamic efficiency and threatens the operation of generating units. In this research paper, a slagging diagnosis method based on analysis of vibration signals from tube panels is proposed to monitor slagging conditions. We fabricated the tube panel tube according to the actual structure of the heating panel in the laboratory to study the relationship between the vibration signal and various slagging conditions and air velocity. Root Mean Square (RMS) in the time field and waveform breakdown in the frequency field are used to extract features from the vibration signal and predict slagging conditions without being turned off. It was found that the RMS value of the panel tube signal decreased with increasing slagging weight, especially at low air speeds. The relative signal energy at a certain frequency will experience a significant change after the panel tube is slagged. To verify the experimental results on changes in the panel tube vibration signal under various slagging conditions, we succeeded in demonstrating our laboratory results through analysis of the vibration signal from the heating panel tube at PLTU Unit 3 Paiton, Probolinggo Regency, East Java. This shows that the vibration signal between the heater and the boiler wall can be collected and used for the diagnosis of slagging in a running coal boiler. Our study is promising for the prediction of slagging and further mitigating the risk caused by slagging of exchange panels in boilers

Numerical models for thermochemical degradation of thermally thick woody biomass, and their application in domestic wood heating appliances and grate furnaces

This paper reviews the current state-of-the-art of numerical models used for thermochemical degradation and combustion of thermally thick woody biomass particles. The focus is on the theory of drying, devolatilization and char conversion with respect to their implementation in numerical simulation tools. An introduction to wood chemistry, as well as the physical characteristics of wood, is also given in order to facilitate the discussion of simplifying assumptions in current models. Current research on single, densified or non-compressed, wood particle modeling is presented, and modeling approaches are compared. The different modeling approaches are categorized by the dimensionality of the model (1D, 2D or 3D), and the one-dimensional models are separated into mesh-based and interface-based models. Additionally, the applicability of the models for wood stoves is discussed, and an overview of the existing literature on numerical simulations of small-scale wood stoves and domestic boilers is given. Furthermore, current bed modeling approaches in large-scale grate furnaces are presented and compared against single particle models.acceptedVersio

Additives to Mitigate against Slagging and Fouling in Biomass Combustion

A crucial part of current and future energy strategy involves the replacement of coal with biomass. However, the composition of biomass creates operational issues in large scale combustion, potentially creating severe slagging and fouling deposition and in worst case scenarios boiler shutdown. Additives have shown promise in improving the deposition of biomass ashes, by altering the composition of the ash and subsequently improving the melting behaviour and deposit properties. However, the effect of additives upon the performance of large scale combustion boilers is not fully understood. This research focuses on the impact of two promising aluminosilicate-based (Al-Si) additives, coal pulverised fuel ash (PFA) and Kaolin powder (KAO), upon the ash properties of different biomass types (olive cake, white wood, bagasse and a power station fly ash). The first area of research was to determine the effect of Al-Si additives upon the electrical resistivity of the ashes across a range of temperatures, which can have a significant impact upon electrostatic precipitator (ESP) performance. A bespoke resistivity test was designed and built for this purpose, based on existing standards. Results showed that biomass ash resistivity is typically lower than that of coal ash by an order of magnitude or more. In some cases, the resistivity may be low enough to cause operational problems and increased particle emissions. The use of additives resulted in increased resistivities, thereby reducing the risk of lower ESP collection efficiencies. Although ESP loads would be increased, this would not be expected to negatively impact emissions. Analysis of the ash compositions indicated that, contrary to previous experience with coal ashes, potassium concentration is an important factor in biomass ash resistivity, meaning that current predictive models are inadequate for biomass and biomass-additive compositions. Therefore, an existing model has been modified using the experimental data and taking into account potassium concentration; this produced reasonable predictions, and showed promise in predicting the resistivity of both biomass and coal ashes. The second area of research was focused upon ash melting behaviour. High temperature viscosity was used to determine ash flow behaviour at temperatures encountered in and around the combustion zone of large scale boilers. Results showed that KAO use with high potassium, high chlorine olive cake (OCA) would significantly improve the flow properties of the ash at combustion temperatures, resulting in ideal viscosities and significantly improved slagging deposition. Thermodynamic modelling data indicated that this was due to the decreased concentrations of magnesium and phosphorus and increased alumina concentrations within the ash, resulting in the formation of high melting temperature minerals and compounds. Ash fusion testing further indicated that KAO can significantly increase flow temperatures of biomass: however, PFA was observed to be less effective. In the case of high silica biomass, PFA was found to have a significant adverse effect upon flow temperature, which would lead to significantly worse slagging. Sinter strength testing was investigated across a temperature range of 800-950°C. Both additives were found to improve the properties of OCA by binding potassium as silicates and aluminosilicates. This eliminates severe sintering caused by KCl sublimation and fluxing at 850°C by binding potassium as silicates and aluminosilicates. However, for the other biomass sinter strengths were increased with additive use. Although most results were below the strengths required for soot blower removal, high additive concentrations produced problematic sinter strengths. It was determined that kaolin has the greatest potential as an additive to reduce deposition issues from biomass combustion, due to its high kaolinite content. Coal PFA was determined to be less effective due to its high mullite and iron concentration. Finally, results indicated that Al-Si additive use is unsuitable for biomass containing high levels of SiO2, and should be used only on biomass with either low SiO2 or high KCl concentrations. However, lower additive rates need to be investigated in future